Dewormers and other medications can dehydrate beardies and also kill off the good flora in their gut. So make sure your little guy is getting plenty of extra
hydration, either from baths or oral fluids (misting doesn't do much since they don't absorb water through their skin).
To help his tummy, you can pick up a probiotics supplement online or from a pet store. Some probiotics supplement product names are Benebac or Acidophiluz; there are others out there as well. You can also just use non-dairy soy yogurt (beardies can't digest dairy too well). Make sure to leave 6 hours between giving meds and probiotics though, or the good flora will get killed off right away. The probiotics in the supplements or the yogurt will help put some of the good flora back in his tummy, which will help his appetite and digestion.